The invention relates to high intensity discharge lamps, such as high pressure mercury vapor or metal halide lamps. In lamps of this type, the arc is produced in an inner discharge tube which is enclosed by an outer envelope made of a glass which is opaque to ultraviolet radiation produced by the lamp. If the outer envelope is broken, such lamps may constitute a health hazard since prolonged exposure to the ultraviolet radiation may cause burns, similar to intense sunburns, to the skin of people in the vicinity of the lamp.
Several approaches have been proposed for extinguishing the lamp when the outer envelope is ruptured. One such approach is to include a mechanical switch which opens and interrupts the supply of electrical power to the lamp upon rupture of the outer envelope. Such switches responded to either a change in the pressure within the outer envelope or are of the spring type which are held in the closed position by the outer envelope. The disadvantage of such arrangements is that it requires relatively complex and expensive mechanisms. Moreover, for proper operation of the spring type switch, the break in the outer envelope must occur at the location of the switch.
Another approach that has been proposed is to use a tungsten filament as a fusible safety link. Upon breakage of the outer envelope, the tungsten filament will oxidize and fail when it comes into contact with air. The disadvantage of this arrangement is that the tungsten filament must be maintained at a very high temperature of 1200.degree. to 1800.degree. C. during operation of the lamp. Such a tungsten fusible link, therefore, consumes an appreciable portion of the total power consumed by the lamp.
It is the object of the invention to provide a safety switch which extinguishes the lamp upon rupture of the outer envelope without the aforementioned disadvantages of the prior art construction.